Means of expanding the spectral wavelength region of silver halide photographic emulsions (hereinafter "silver halide emulsions" or simply "emulsions") and improving their sensitivity are known as spectral sensitization techniques. Spectral sensitizing dyes used for such purposes are known to include a large number of compounds such as cyanine dyes and merocyanine dyes.
These spectral sensitizing dyes must be able not only to expand the spectral wavelength region of silver halide emulsions but also to satisfy the following various requirements.
1) They have appropriate spectral sensitizing regions. PA1 2) They have a high spectral sensitization efficiency. PA1 3) They cause no fog and have no ill influence on the characteristic curve, e.g., changes of gamma. PA1 4) They cause no changes in photographic performance such as fog when light-sensitive materials containing sensitizing dyes are aged, in particular, when stored in an environment of high temperature and high humidity. PA1 5) They cause no color cross-over because of diffusion of a sensitizing dye to a layer having a different spectral wavelength region. PA1 6) They cause no dye-stain after developing, fixing and washing.
However, none of spectral sensitizing dyes hitherto disclosed have attained the level high enough to well satisfy all of these requirements. In particular, as developing time has been made shorter and washless processing or processing solution recycling has become usable, dyes have become likely to remain in processing solutions.
As a result, color stain has come to appear in light-sensitive materials having been processed (hereinafter "residual color stain"), causing the problem of a great fall in values of commercial products. In other words, how better prevent the residual color stain in spectral sensitizing dyes has come into question as an important technical problem.
As spectral sensitizing dyes capable of achieving spectral sensitization in the red-light wavelength region, those which are known to be effective are exemplified by complex cyanine dyes or complex merocyanine dyes disclosed in Belgian Patent No. 541,245, U.S. Pat. No. 2,493,747, No. 2,493,748, No. 2,743,272 and No. 3,335,010, French Patent No. 2,113,248, German Patents No. 1,024,800, No. 2,153,570 and No. 2,300,321 and Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 171135/1991; cyanine dyes disclosed in Japanese Patent O.P.I. Publications No. 11121/1974, No. 33622/1976, No. 115821/1976, No. 115822/1976, No. 72937/1983, No. 203446/1986, No. 256054/1990 and No. 15042/1991; and merocyanine dyes disclosed in U.S. Pat. No. 2,493,747, No. 2,493,748 and No. 2,519,001 and Japanese Patent O.P.I. Publications No. 106422/1976 and No. 214030/1984.
Some of these dyes have been made less ascribable to residual color stain by introducing a water-soluble group into the molecule, but can not be well effective for its prevention or have the problems that spectral sensitivity becomes lower and sensitivity variations tend to occur as coating solutions are aged or stand with time. Thus, they still can not be satisfactory.
Dyes disclosed in recent years in European Patents No. 363,104 and No. 363,107 are seen to have been improved in residual color stain proofness, but have the problem that photographic performance may vary and deteriorate when light-sensitive materials having been subjected to spectral sensitization are left to stand for their storage.
Hithertofore, it is well known to carry out spectral sensitization using two or more kinds of spectral sensitizing dyes in combination for the purpose of improving spectral sensitivity in the red-light wavelength region as in the case of the present invention. For example, they are typified by specific thia- or selenacarbocyanine dyes. Such spectral sensitization, however, can not better prevent the residual color stain.
Japanese Patent O.P.I. Publications No. 18726/1979, No. 135461/1984, No. 246054/1987, etc. disclose a technique in which a supersensitizer is used together to increase spectral sensitivity so that a load of residual color stain can be decreased. This, however, is still at an unsatisfactory level, and it has been sought to make a further improvement.